The electrolytic recovery of metals, especially metals that are more noble than hydrogen, takes place from an aqueous solution of the metal. The recovery of zinc from an aqueous solution can also be performed electrolytically, although zinc is a less noble metal than hydrogen. It is typical of the method that a pure metal is reduced from the solution onto the cathode and a gas forms on the anode, which depending on the conditions is chlorine, oxygen or carbon dioxide. Insoluble anodes are used as the anode. In this case electrolysis is called electrowinning. The most common metals that are produced by electrowinning from an aqueous solution containing sulphuric acid are copper and zinc. The potential in the copper and zinc electrolysis process is regulated to a range in which oxygen is formed at the anode.
Producing a pure metal in electrolysis is the sum total of many factors, but one important factor is the quality of the anode. The anodes used in copper and zinc electrowinning are usually made of lead or lead alloy, where the alloy contains 0.3-1.0% silver and possibly 0.04-0.07% calcium. When the lead based anode described above is used for example in zinc electrolysis, in which the H2SO4 concentration is of the order of 150-200 g/l, the lead of the anode starts to dissolve and precipitate on the cathode. The precipitation of lead on the cathode also causes short circuits, which impede electrolysis.
Under electrolysis conditions, a layer of lead oxide is formed naturally on the surface of the lead anode, which partially protects the anode from corrosion. In addition, zinc electrolyte usually contains 3-6 g/1 manganese, which over time precipitates a layer of MnO2 on the anode surface. However, when there is a thick layer of MnO2 on the surface of the anode, the anode starts to behave as if it was an MnO2 electrode. The drawbacks of a naturally forming layer of MnO2 are that a thick layer may cause short circuits and part may fall into the electrolyte, if its adhesiveness is poor in places. A solid MnO2 layer is believed to have its own effect on the corrosion of lead anodes and so the precipitation of manganese ions from the electrolyte solution is considered undesirable. A major disadvantage is also that a thick MnO2 layer requires a high anode potential to form oxygen and this raises the energy costs of the process.
Attempts have been made to prevent anodes from corroding in many ways. One way to solve the problem is to form a catalyst layer on the surface of the anode before submersing the anode in the electrolyte, so that the layer protects the anode from corrosion. However, finding a suitable catalyst causes difficulties, because electrolysis operates at fairly high acid concentrations.
Particularly in chlorine-alkali electrolysis, anodes known as dimensionally stable anodes (DSA), which are described for example in U.S. Pat. Nos. 3,632,498 and 4,140,813, have been used for decades. These have also been proposed for use instead of lead electrodes in the electrolysis of zinc and copper because of their energy-saving characteristics, but traditional anodes made of lead alloy are nevertheless still in use in the majority of the world's copper and zinc electrolysis facilities.
Methods are known in which an electrocatalyst is formed on the surface of DSA electrodes. The electrode material, which is usually titanium, is pretreated by etching or sandblasting and can be given further after-treatment by spraying some kind of valve metal such as titanium or its oxide. The final catalytic coating is formed from a solution or suspension of the catalyst or its precursor, such as a metal salt or organometallic compound. These chemicals are generally decomposed thermally i.e. treated in a furnace at a raised temperature to form the desired, catalytically active surface. The catalyst material is a metal or oxide of the platinum group or alternatively one of the following metals: titanium, tantalum, niobium, aluminium, zirconium, manganese, nickel or an alloy thereof. The catalyst layer can be produced on the surface in different ways, such as painting on or by spraying, but the layer formation requires one or several heat treatments at a temperature between 450-600° C. Often further intermediate layers are formed on the electrode surface before the formation of the final protective layer. These kinds of methods are described in e.g. EP patents 407349 and 576402 and U.S. Pat. No. 6,287,631.
A method is described in U.S. Pat. No. 4,140,813, in which a titanium oxide layer is formed on a sandblasted titanium anode by plasma or flame spraying, where the composition of the layer can be affected by means of the spraying temperature and composition of the gas used. In plasma and flame spraying the coating material melts during spraying. The oxide layer that is formed i.e. the electrically conductive substrate layer is further treated with an electrochemically active substance. As activation substances, platinum metals are employed, preferably ruthenium or iridium, as elements or as compounds and they are brushed on top of the oxide layer.
Coatings have also been developed for the surface of a lead anode to protect it and facilitate the development of oxygen. An anode is described in U.S. Pat. No. 4,425,217, Diamond Shamrock Corp., in which the base of lead or lead compound is provided with catalytic particles of titanium, which contain a very small amount of platinum group metal or an oxide thereof. In the coating fabrication method both the anode and the titanium powder are treated by etching and the powder is heat-treated in order to oxidize the precious metal salts into oxides. The powder is attached to the anode surface by pressing.
EP patent 87186, Eltech Systems Corp., presents a means of providing a catalyst used on the surface of a DSA electrode on the surface of a lead anode, in which the catalyst is formed from a titanium sponge, which is equipped with ruthenium-manganese oxide particles. The making of the catalytic coating mentioned above in the environment of a zinc and copper electrolysis facility seems quite difficult and the coating becomes fairly costly. Attaching the powder to the surface of the anode also occurs by pressing.